Liquid crystal display devices on a basic level comprise two pieces of polarized glass having a polarizing film side and a glass side. A special polymer that creates microscopic grooves (oriented in the same direction as the polarizing film) in the surface is rubbed on the non-polarizing film side of the glass. A coating of nematic liquid crystals is added to one of the filters. The grooves cause the first layer of molecules of the liquid crystals to align with the filter's orientation. The second piece of glass is added with the polarizing film at a right angle to the first piece. Each successive layer of liquid crystal molecules gradually twists until the uppermost layer is at a 90 degree angle to the bottom, thus matching the orientation of the second polarized glass filter.
As light strikes the first filter, it is polarized. If the final layer of liquid crystal molecules is matched up with the second polarized glass filter, then the light will pass through. The light which passes through is controlled through the use of electric charges to the liquid crystal molecules.
Active-matrix LCDs depend on thin film transistors (TFT). Basically, TFTs are tiny switching transistors and capacitors arranged in a particular matrix on the glass substrate. These TFTs control which areas receive a charge and therefore, the image seen by the viewer.
The light to the LCD device may be supplied through the use of a backlight unit. Two possible backlight unit types include cold cathode fluorescent lamps (CCFLs) and external electrode fluorescent lamps (EEFLs).
FIG. 4A illustrates a conventional external electrode in which metal capsules are bonded at the end of the glass tubes, and ferrodielectrics are applied to the inside of the metal capsules. This type of electrode is disclosed in U.S. Pat. No. 2,624,858 to Greenlee. However, the bonded portions of the electrodes can be easily damaged since the coefficient of the thermal expansion of the glass tubes is different from that of the metal capsule.
FIG. 4B illustrates another type of electrode, which is disclosed in U.S. Pat. No. 6,674,250 to Cho et al. The electrodes of Cho et al. are metal caps attached to the sealed glass tube by using conductive adhesives 16. In the same disclosure, the electrodes can also be conductive tapes 14 with adhesives wherein the tapes 14 are attached to the glass tubes 2, as shown in FIG. 4C.
FIG. 4D illustrates another type of electrode, which is disclosed in U.S. Pat. No. 6,914,391 to Takeda et al. The electrodes disclosed in Takeda et al. are aluminum foils 15, attached to the sealed glass tube 2 by using an electrically conductive silicone adhesive layer.
The use of adhesives, as in the prior art EEFLs mentioned above, has the disadvantage of creating weak bonds between the electrode and the glass tube of the EEFL device. The adhesives provide only mechanical bonding and the weak bonding of the electrodes may result in poor reliability performance. For example, gaps between the electrodes and the glass tubes may appear during thermal cycles due to the mismatching of thermal expansion coefficients between the metal caps (electrodes) and glass tubes. Gaps may also appear when the adhesives deteriorate in harsh environments. Gaps between the electrodes and the glass tubes can lead to EEFL failures because the high operating voltage of EEFL would not be uniformly applied to the glass tubes. Higher electrical resistance around the gaps leads to destructive damages to the glass tubes. Also, the higher stress around the gap can also intensify the separation and accelerate the failure of the device during the reliability testing.
The present invention is a novel method for forming the electrodes of an EEFL and for forming an LCD device. The invention relates to a fluorescent lamp with external electrodes and method(s) of forming such lamps and electrodes, wherein such methods and electrodes utilize thick film pastes, and backlight units formed from the methods described herein with particular utility in LCD applications.